What is endorphin
Subjects and Methods SubjectsĮxperiments were performed in male Wistar rats (200–250 g bred at the Charité-Universitätsmedizin Berlin, Berlin, Germany) in accordance with standard ethical guidelines and approved by the local authorities (Landesamt für Arbeitsschutz, Gesundheit und Technische Sicherheit, Berlin, Germany). To this end we studied 1) POMC, CPE, PC1, PC2, 7B2, and END proteins by Western blotting 2) the distribution of POMC, CPE, PC1, PC2, 7B2, and END immunoreactivity within cells of inflamed sc paw tissue and blood by single and double immunohistochemical techniques 3) the subcellular distribution of END using immunoelectron microscopy and 4) END release from immune cells upon incubation with noradrenaline in vitro. In this study we examine whether the components of this classical secretory pathway are also present and functional within inflammatory cells. The subsequent secretion of END requires secretory granules deriving from the Golgi network for transport to the cell membrane ( 18). The inactive pro-PC2 is bound to 7B2 (a chaperone-like binding protein) and is transported from the ER to later compartments of the secretory pathway, where it matures to active PC2 ( 16) thereafter, PC2 converts βLPH into β-MSH and END ( 17). PC1/3 mediates the initial cleavage at paired basic residues into ACTH and β-lipotropic hormone (βLPH) ( 13, 15). Two mammalian prohormone convertases, PC1 (also called PC1/3) and PC2, cleave POMC and other peptide precursors within the TGN ( 13, 14). The POMC prohormone is directed to the regulated secretory pathway at the TGN by binding to a sorting receptor, identified as (membrane-bound) carboxypeptidase E (CPE) ( 12). In the pituitary gland of adult mammals, POMC processing begins as the nascent polypeptide chain enters the endoplasmic reticulum (ER) directed by the signal peptide ( 9), and POMC cleavage begins in the trans-Golgi network (TGN) ( 10, 11). In contrast to the extensively studied classical posttranslational processing of POMC in the pituitary gland, little is known regarding END processing and pathways of release from immunocytes under inflammatory conditions.
POMC is synthesized mainly in the neurointermediate lobe of the pituitary gland ( 8) and is the precursor of END and other peptides ( e.g. Environmental stimuli (stress) and releasing agents (CRH, cytokines, and catecholamines) can activate these immune cells to secrete END ( 5, 6) and to inhibit inflammatory pain via interaction with peripheral opioid receptors ( 6, 7). Our previous immunohistochemical and in situ hybridization studies demonstrated the presence of proopiomelanocortin (POMC) mRNA and END in rats with painful paw inflammation ( 4). UNDER INFLAMMATORY CONDITIONS, various types of immune cells have been shown to produce and contain opioid peptides in culture ( 1, 2) and in situ ( 3, 4). Taken together, our results indicate that immune cells express the entire machinery required for POMC processing into functionally active peptides such as END and are able to release these peptides from secretory granules. Finally, END is released by noradrenaline from immune cells in vitro. Immunoelectron microscopy revealed that END is localized within secretory granules packed in membranous structures in macrophages, monocytes, granulocytes, and lymphocytes.
Using immunohistochemistry we detected END and POMC alone or colocalized with PC1, PC2, carboxypeptidase E, and 7B2 in macrophages/monocytes, granulocytes, and lymphocytes of the blood and within inflamed sc paw tissue.
The aim of the present study was to examine the presence of POMC, carboxypeptidase E, the prohormone convertases 1 (PC1), and 2 (PC2), PC2-binding protein 7B2, and the release of END from inflammatory cells in rats. However, the subcellular pathways of POMC processing and END release have not yet been delineated in inflammatory cells. During stressful stimuli END is released and interacts with peripheral opioid receptors to inhibit pain.
The opioid peptide β-endorphin (END) as well as mRNA for its precursor proopiomelanocortin (POMC) are found not only in the pituitary gland, but also within various types of immune cells infiltrating inflamed sc tissue.